The invention generally relates to a technique and system for performing a cross well survey.
Well stimulation, injection and production can lead to a change in the stress state of the formation, resulting in rock failure and stress release. This stress release can generate both compressional and shear-radiated seismic energy. The nature and characteristics of detected seismic energy can reveal information about the failure mechanism and the amount of stress released.
By detecting a microseismic event by one or more multi-component sensors, it is possible to triangulate the location of the event. This can reveal information about the geometry and extent of the pressure disturbance, the geometry of fracture growth, and the principal stress direction of the formation.
Microseismic hydraulic fracture monitoring, in general, has two principal applications. The first application is real-time microseismic event detection and location for real-time hydraulic fracture monitoring. This enables the well service stimulation specialist to monitor and adjust the hydraulic fracture operation based on the observed and modeled extent of fracture growth. The second application is post job analysis and interpretation. The microseismic event data may be processed and interpreted to better understand the mechanisms and geometry of fracture growth and the current stress state of the formation. The specialist can use this information in the planning of further well stimulation and possible in-fill drilling for enhanced production.
The analysis and processing of microseismic data requires high fidelity recording of the microseismic events as a function of elapsed time; and detailed knowledge of the subsurface velocities, compression and shear, and the amount and nature of velocity anisotropy in the rock layer. One way to determine the amount and nature of velocity anisotropy is through the use of walkaway vertical seismic profile (VSP) measurements. However, these measurements may be too costly or may be impractical for a rugged terrain. Additionally, velocity inversions in the subsurface may additionally limit arrival angles and so not give a complete picture of the anisotropy present in the subsurface.
Thus, there exists a continuing need for better ways to build and/or calibrate a seismic velocity model.